Identify the model number of your Tube Screamer pedal. The essential change to the electronics is to the values of two resistors in the output section of the pedals. The various Ibanez circuit boards place these resistors in different places.
Use the model number to locate the resistors on your pedal's circuit board. The resistor numbers are printed on the circuit board. To modify a TS9 to TS-808 specs, use the 100 ohm resistor to replace R1, and the 10k resistor to replace R2. For the TS5: 100 ohm replaces R34; 10k replaces R35. For the TS7: 100 ohm replaces R55; 10k replaces R58. For the TS9 and TS-808 reissues: 100 ohm replaces R15; 10k replaces R13.
Remove the solder from both sides of the target resistor, taking care not to overheat the board or to contact surrounding components. Insert the new resistor in its place. Solder the leads with similar precautions and trim the extra lead length.